For the clarification of medical questions or for interventional radiology, X-ray imaging devices, for example a C-arm X-ray device or a computed tomography device, are being used ever more frequently. It is common to X-ray imaging devices that they have an X-ray radiation source, typically an X-ray tube, and an X-ray radiation detector cooperating with the X-ray radiation source. The X-ray radiation emitted by the X-ray radiation source passes through a patient to be investigated and is attenuated by interaction with the different tissue types of the patient. The detector is arranged behind the patient in relation to the X-ray radiation source, absorbs the X-ray radiation remaining behind the patient and converts it into the electrical signal corresponding to the X-ray attenuation caused by the patient.
The X-ray radiation source emits X-ray radiation with an emission spectrum, that is, the gamma quanta emitted by the X-ray radiation source typically have an energy distribution comprising a plurality of quantum energy values. The X-ray radiation emitted is therefore polychromatic. The emission spectrum is substantially influenced by the X-ray tube voltage or the acceleration voltage with which the X-ray radiation source is operated. Generalizing, it can be stated that the higher the acceleration voltage is, the greater also is the mean X-ray quantum energy of the emission spectrum.
It is known that different materials or tissue types, for example, water or bone interact to different extents with X-ray radiation. Simply expressed, the image contrast of all X-ray images is based on these differences. Furthermore, the energy-dependency of the X-ray attenuation when passing through material is also known. Lower energy X-ray radiation is more strongly absorbed by matter than higher energy X-ray radiation. These differences in the interaction of X-ray radiation with matter must be taken into account in X-ray imaging in order to generate X-ray image recordings which have sufficient image quality to answer the medical questions and also to protect the patient against unnecessary dosage exposure.
Depending on the case, superimposition images or sectional images of an object or a patient can be generated with an X-ray imaging device. In order to generate classic X-ray superimposition images, the object to be imaged is irradiated by an X-ray radiation source in one direction and imaged on an X-ray film or via an X-ray radiation detector. A projection of the imaged volume is created on a surface. Image parts of the irradiated volume lying one after another in the radiation direction become superimposed. Whether the X-ray attenuation visible in the superimposition image was caused by a material of relatively great X-ray absorption or by a greater layer thickness cannot be distinguished from the image.
A topogram, for example, which is also known as a scout view corresponds to such a classical, two-dimensional X-ray superimposition recording. It measures the individual X-ray attenuation distribution of a patient in the particular projection direction, typically in a lateral or anterior-posterior direction and forms this on the basis of different gray values.
For the generation of a sectional image, X-ray attenuation profiles of an object are measured from many different projection directions and therefrom, the three-dimensional volume structure of the object is reconstructed. By way of a computer-assisted image reconstruction which can be carried out, for example, with the algorithm of the filtered, weighted back projection, for each volume element of the object, a so-called voxel, the X-ray attenuation level is determined and therefrom, the sectional image is calculated. The sectional image corresponds to a transverse section through the examination object, wherein the section typically lies in a plane parallel to or substantially parallel to the X-rays used for generating the projection data. With a plurality of successive circulations of the X-ray radiation source round the object, mutually adjacent sectional images can be generated. A plurality, for example, several hundred individual images together form a volume rendering of the object.
An advantage of computed tomography, abbreviated to CT as compared with conventional projection X-ray imaging, is that superimposition-free representations can be generated. Imaging precision and detail accuracy are correspondingly higher in CT sectional images. In some cases however, apart from a CT data set, an additional X-ray projection is desirable. With the aid of a topogram, for example, the X-ray dose for a CT scan can be adapted particularly well to a patient. On the other hand, a trained person can draw from the X-ray projection particular information concerning the imaged object “at a glance” which could only be drawn from a CT data set typically containing a large quantity of data at the cost of much effort and time.